The present invention relates to an optical fiber base material as the base of an optical fiber, its manufacturing method and the optical fiber.
In a single mode optical fiber, there is a so-called depressed clad type structure having a refractive index distribution as shown in FIG. 1B to improve chromatic dispersion characteristics. Namely, as shown in FIG. 1A, a first clad portion 3 is formed outside a core portion 2 in this single mode optical fiber 1. Further, a second clad portion 4 having a refractive index higher than that of the first clad portion 3 is formed outside this first clad portion 3.
The present invention in one aspect provides the following optical fiber base material. Namely, the optical fiber base material comprises:
a core original layer formed as a core portion of an optical fiber; and
a first clad original layer and a second clad original layer sequentially laminated and formed on an outer circumferential face of the core original layer;
wherein chlorine is included in both the first and second clad original layers, and
the second clad original layer includes chlorine of a concentration higher than that of the first clad original layer within a concentration range not less than 500 ppm and not more than 5000 ppm.
The invention in another aspect also provides the following optical fiber. This optical fiber comprises:
a core portion; and
first and second clad portions sequentially laminated and formed on an outer circumferential face of the core portion;
wherein chlorine is included in both the first and second clad portions, and
the second clad portion includes chlorine of a concentration higher than that of the first clad portion within a concentration range not less than 500 ppm and not more than 5000 ppm.
The invention in another aspect further provides the following manufacturing method of the optical fiber base material. This manufacturing method of the optical fiber base material comprises the steps of:
a process for forming a porous glass intermediate body constructed by depositing a porous first clad original layer on the circumferential surface of a porous core original layer;
a process for subsequently arranging and consolidating the porous glass intermediate body within an atmospheric gas including a chlorine-based gas with fluorine compound;
a process for making a porous optical fiber base material by forming a porous second clad original layer on the circumferential surface of a transparent glass intermediate body made by the consolidation process mentioned above;
a process for subsequently dehydrating the porous optical fiber base material at a set temperature for dehydration to maintain a porous state of the second clad original layer within the atmospheric gas having a chlorine concentration higher than that of the atmospheric gas at the time of consolidation of the porous glass intermediate body; and
a process for making the optical fiber base material formed as transparent glass by consolidating the porous optical fiber base material within an atmospheric gas having a chlorine concentration higher than that of the atmospheric gas during the consolidation of the porous glass intermediate body.